1. Field of the Invention
The present invention relates to a projection zoom lens suitable for use on a projector for projecting an image formed on a film, a slide or a liquid crystal display in an enlarged picture on a screen.
2. Description of the Related Art
Most projectors for projecting an image formed on a film, a slide or a liquid crystal display in an enlarged picture on a screen have a projection optical system employing a telecentric zoom lens.
Most prior art telecentric zoom lenses, such as those disclosed in JP-A Nos. 137165/2000, 186235/1998, 206409/2000, 019400/2000, 190821/1999, 168193/1998, 243917/1997 and 231219/1999, have a half filed angle of about 25xc2x0 when set for the largest angular field.
A telecentric zoom lens having a half field angle of about 30xc2x0 disclosed in, for example, JP-A No. 206409/2000 uses an aspherical lens as a comparatively large lens of the first lens group or the last lens group to reduce aberrations resulting from increase in the field angle, particularly, a distortion.
Projection zoom lenses have a zoom ratio on the order of, for example, 1.4, and most projection zoom lenses are of a two-lens-group type, three-lens-group type, four-lens-group type or five-lens-group type.
In the projection zoom lens, the first lens group has a high power to receive light rays in a wide field angle and hence tends to cause a distortion.
A wide-angle telecentric zoom lens employs an aspherical lens to reduce distortion. The aspherical lens is disposed in the vicinity of the front lens on the side of a screen or the rear lens on the side of an image plane to reduce distortion. Thus, the conventional wide-angle telecentric zoom lens is provided with an aspherical lens disposed on the exit or the entrance side thereof to reduce distortion.
Sometimes, an aspherical lens exercises not only a function to reduce distortion but also a function to reduce other aberrations excluding a chromatic aberration.
However, since the aspherical lens is disposed on the exit side or the entrance side in the conventional telecentric zoom lens, the design of the telecentric zoom lens is aimed principally at only the reduction of distortion and hence the telecentric zoom lens is unable to reduce other aberrations satisfactorily.
Accordingly, it is an object of the present invention to solve the foregoing problems in the prior art projection zoom lens and to provide a wide-angle projection zoom lens of simple construction including a limited number of lenses and capable of satisfactorily reducing aberrations, and to provide a projector which has high quality image.
To achieve the object, the present invention provides a projection zoom lens including a first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a negative refracting power, and a fourth lens group having a positive refracting power; wherein the first, the second, the third and the fourth lens groups are arranged in that order from a screen side toward an image plane side along an optical axis; the first and the fourth lens groups are kept stationary, and the second and the third lens groups are moved along the optical axis to vary power from a limit wide angle power toward a limit telephoto power; the second lens group includes a 2nd-group 1st positive lens, a 2nd-group 1st compound lens formed by joining a 2nd-group 2nd positive lens having a convex surface on the image plane side and a 2nd-group 1st negative lens having a concave surface on the screen side, and a 2nd-group 3rd positive lens arranged in that order from the screen side toward the image plane side; the third lens group includes a 3rd-group 1st meniscus lens having a concave surface on the image plane, a 3rd-group 1st compound lens formed by joining a 3rd-group 1st negative lens having opposite concave surfaces and a 3rd-group 1st positive lens having opposite convex surfaces, and a 3rd-group 2nd meniscus lens having a convex surface on the image plane side arranged in that order from the screen side toward the image plane side; the first lens group has a focal length f1, the 2nd-group 1st positive lens and the 2nd-group 1st compound lens have a composite focal length f2AB of a sign opposite to that of the focal length f1, and the focal length f1 and the composite focal length f2AB meet an inequality:
0.5 less than |f1/f2AB| less than 2.0.
In the projection zoom lens according to the present invention, distance d between one surface on the side of the image plane of the 2nd-group 3rd positive lens and one surface on the side of the screen of the 3rd-group 1st meniscus lens varies between a value dW for the limit wide angle power and a value dT for the limit telephoto power, and the values dW and dT satisfy an inequality:
2.0 less than dT/dW less than 4.0.
In the projection zoom lens according to the present invention, the 2nd-group 3rd positive lens and the 3rd-group 1st meniscus lens are disposed at positions in the vicinity of an aperture position where a principal ray emerging from an image plane intersects the optical axis.
In the projection zoom lens according to the present invention, the 3rd-group 1st meniscus lens is an aspherical lens.
In the projection zoom lens according to the present invention, all the lenses are spherical lenses.
In the projection zoom lens according to the present invention, the second lens group includes a first lens subgroup and a second lens subgroup arranged in that order from the screen side toward the image plane side, the first lens subgroup has the 2nd-group 1st positive lens, and the second lens subgroup has the 2nd-group 2nd positive lens, the 2nd-group 1st compound lens and the 2nd-group 3rd positive lens.
In the projection zoom lens according to the present invention, the focal length f1 of the first lens group and the composite focal length f2AB of the 2nd-group 1st positive lens and the 2nd-group 1st compound lens have opposite signs, respectively, and meet an inequality: 0.5 less than |f1/f2AB| less than 2.0, and the composite power of the 2nd-group 1st positive lens and the 2nd-group 1st compound lens on the screen side of the second lens group, is substantially equal to that of the first lens group and has a sign opposite to that of the power of the first lens group. Therefore, distortions produced by the first and the second lens group cancel each other even if the first lens group has a large power. When |f1/f2AB| is not smaller than 2.0 or not greater than 0.5, it is difficult to limit the distortion which is liable to be produced when the projection zoom lens is set for a wide angle power to a small value and other aberrations, such as curvature of field and coma aberration, increase.
The 2nd-group 1st compound lens of the second lens group functions as an achromatic lens for reducing chromatic aberration of magnification.
Whereas the 2nd-group 3rd positive lens of the second lens group, and the 3rd-group 1st meniscus lens of the third lens group move independently of each other, the surface on the image plane side of the 2nd-group 3rd positive lens, and the 3rd-group 1st meniscus lens are comparatively close to each other, and the positional relation between the surface on the image plane side of the 2nd-group 3rd positive lens, and the 3rd-group 1st meniscus lens meet the inequality: 2.0 less than dT/dW less than 4.0. The composite focal length of the second lens group is long, the moving distance of the second lens group increases and the length of the projection zoom lens is excessively long when dT/dW is not smaller than 4.0, and the composite focal length of the second lens group is short and aberrations are liable to be produced when dT/dW is not greater than 2.0.
Since the 2nd-group 3rd positive lens and the 3rd-group 1st meniscus lens are disposed at positions in the vicinity of the aperture position where a principal ray emerging from the image plane intersects the optical axis, formation of flare spots, which are liable to be formed when the aperture is large, can be suppressed without requiring disposing an aperture stop at a position where an aperture stop is disposed in the conventional projection zoom lens. Particularly, formation of flare spots when the projection zoom lens is set for the limit wide-angle power can be effectively suppressed. The term, aperture position signify a position suitable for disposing an aperture stop, corresponding to the intersection of a main light beam emerging from the image plane and the optical axis. The positions of the 2nd-group 3rd positive lens and the 3rd-group 1st meniscus lens the aperture position are nearer than those of other lenses and are within a distance equal to several times the thickness of the lens, such as three times the thickness of the lens, from the intersection of the principal ray and the optical axis.
The distance d between the surface on the image plane side of the 2nd-group 3rd positive lens and the surface on the screen side of the 3rd-group 1st meniscus lens increases as the power is changed from the limit wide angle power toward the limit telephoto power, and the 2nd-group 3rd positive lens and the 3rd-group 1st meniscus lens can be disposed at the positions in the vicinity of the aperture position when the distances dW and dT meet the inequality: 2.0 less than dT/dW less than 4.0. Thus, formation of flare spots, which are liable to be formed when the aperture is large, can be suppressed without requiring disposing an aperture stop at a position where an aperture stop is disposed in the conventional projection zoom lens and, particularly, formation of flare spots when the projection zoom lens is set for the limit wide angle power can be effectively suppressed.
Aberrations can be further efficiently improved by using an aspherical lens as the 3rd 1st meniscus lens. As compared with the use of an aspherical lens as the lens in the vicinity of the front lens nearest to the screen or the lens in the vicinity of the rear lens nearest to the image plane, the object of employment of the aspherical lens is not aimed only at reducing distortion, and the object of use of the spherical lens is not limited to the reduction of distortion, and can be used for reducing other aberrations including spherical aberration and astigmatism. Since the aspherical lenses have diameters smaller than those of aspherical lenses as the front and the rear lens, and the lenses in the vicinity of the front and the rear lens may be formed of inexpensive materials, the projection zoom lens can be manufactured at low costs.
Since the second lens group includes the first and the second lens subgroup which are individually movable relative to each other, the first lens subgroup has the 2nd-group 1st positive lens, and the second lens subgroup has the 2nd-group 2nd positive lens, the 2nd-group 1st compound lens and the 2nd-group 3rd positive lens, the inequality: 0.5 less than |f1/f2AB| less than 2.0 can be satisfied without difficulty, the 2nd-group 3rd positive lens and the 3rd-group 1st meniscus lens can be disposed at positions in the vicinity of the aperture position without difficulty, and formation of flare spots liable to be formed when the aperture is increased can be suppressed.
In claims 7, 8, 9, 11 and 12, values modified by xe2x80x9capproximatelyxe2x80x9d are following values. For example, xe2x80x9capproximately 0.94xe2x80x9d in claim 7 indicates a value greater than about 0.93 and smaller than about 0.95. Similarly, xe2x80x9capproximately 3.3xe2x80x9d indicates a value greater than about 3.2 and smaller than about 3.4, xe2x80x9capproximately 1.0xe2x80x9d indicates a value greater than 0.9 and smaller than 1.1, xe2x80x9capproximately 3.1xe2x80x9d indicates a value greater than 3.0 and smaller than 3.2, xe2x80x9capproximately 0.73xe2x80x9d indicates a value greater than 0.72 and smaller than 0.74, xe2x80x9capproximately 2.7xe2x80x9d indicates a value greater than 2.6 and smaller than 2.8, xe2x80x9capproximately 28 mmxe2x80x9d indicates a length greater than 27 mm and smaller than 29 mm, and xe2x80x9capproximately 29xc2x0 indicates an angle greater than 28xc2x0 and smaller than 30xc2x0.
According to the present invention, it is able to provide the projector including an image forming device for forming an image and a projector zoom lens for projecting the image, wherein the projector zoom lens is given by any one of the projector zoom lens mentioned above. The projector is able to provide high quality image. Such the image forming device may be a light modulation device such as a liquid crystal display or a device composed of, for example, micro mirrors, a film or a slide.